Different ways of designing variable focal length lenses based on flat layers of nematic liquid crystals (NLC) are known in the art. Typically, a non-homogeneous electric field is used to induce a suitable configuration of the NLC director (that is, the direction of the preferred molecular orientation) in a cell so as to create a lens-like distribution of the refractive index. Non-homogeneous electric fields can be generated by means of suitable electrode structures provided on one or both cell substrates.
In some cases, a small amount (e.g. up to 3%) of a reactive monomer is added to the NLC. The reactive monomer is substantially uniformly polymerized in situ by uniform UV irradiation during application of the non-homogenous electric field. Polymerization of the monomer in this manner leads to the formation of a spatially uniform polymer network structure or matrix, which reduces the ease with which the NLC director can be reoriented. Accordingly, polymerization of the monomer while the NLC while under the influence of the non-uniform electric field reduces the tendency of the NLC director to re-orient back to its relaxed state when the electric field is removed, thereby producing a “permanent” lens within the NLC. The accuracy of control of the focal length of such a lens depends on the concentration of monomer. Another known method of forming micro-lenses, of fixed focal length, is to use strongly focused light to induce LC reorientation and simultaneous UV photopolymerization.
Different polymer network structures and their influence on the electrical switching properties of NLC have been studied. In particular, for the structure of nematic domains separated by thin polymeric walls, it has been established that the threshold field of nematic reorientation increases as the density of the polymer network increases. Recently, patterned irradiation has been used to produce regions with different threshold voltage for switching. When used in conjunction with a mask in the form of concentric dark and transparent rings, this technique can be used to produce a switchable Fresnel lens.